Rei is an alien heart-throb who is not at all what he seems. Rei is incredibly attractive to most women, but that's only when he's in his normal, humanoid form. To clarify things it should be known that Rei has a strange shape-shifting power. He can transform himself into a goofy looking, gigantic rampaging tiger-bull monster.
Rei is a threat to every man's love life. He has inadvertently broken up countless couples. Although he's handsome most of the time, it is soon apparent that Rei is all looks and no personality. He doesn't seem to have a brain in his head as he is so clueless. He only has one thing on his mind...food. Rei lives to eat and rarely thinks of anything else. He's a non-stop eating machine who is hardly ever seen not stuffing his face, or looking for eats.
Lum is all too familiar with of Rei's shallow personality. He was Lum's former fiance and short-term love interest back on their home planet of Uru, the Oni star. But soon Lum saw him for what he really was and dumped him. Now Lum can't stand Rei, but he still persues her heart which is quite a pathetic display to see. Rei doesn't speak much. Whenever he does speak, he is only capable of uttering one word at a time. This is mostly because Rei doesn't know how to speak Japanese. But also because he's just so incredibly, unbelievably stupid.
Later on in the series Rei hardly ever reverts to his human form and spends most of his time as the tiger-bull monster. When he's like that, girls aren't attracted to him at all. The only person who loves him, even with all of his flaws is Ran but Rei is completely insensitive to her feelings. He eventually begins to cling to her, and they become a couple. It's pretty much only because Ran keeps feeding him and Rei just learns through conditioning that Ran=food. But Ran will take whatever attention she can get.
Notes about the name
The word rei has many meanings. It can mean "spirit" or "greeting" but in his case there's no doubt that his name was meant as "zero."
Voice: Tessho Genda
I don't have much to say about him for lack of evidence. I'm sure Genda is talented, but I find it hard to gauge his ability when Rei does so little talking during the course of the series.